434 Solar Observation. 



SOLAR OBSERVATION.— COLOURS OF STARS.— 



CONSTITUTION OF NEBULA.— TRANSITS 



OF JUPITER'S SATELLITES. 



BY THE REV, T. W. WEBB, A.M., F.R.A.S. 



In a previous paper we enumerated several modes of eliminating 

 the superfluous light and heat otherwise so prejudicial in 

 examining the sun. Two very efficient ones, however, remain to 

 be described. 



One very generally applicable contrivance is a modification 

 of the diagonal eye-piece, which has been long in use, in "Order 

 to avoid the neck-twisting process of observation with an achro- 

 matic at great altitudes. For this purpose a plane speculum is 

 introduced diagonally into the interior of the eye-piece, or pre- 

 ferably into the tube just beyond the eye-piece ; and in the 

 latter form, if a piece of unsilvered glass is used instead of 

 speculum metal, it is obvious that so little light will be reflected 

 from its anterior surface, that a very pale screen-glass will 

 be quite sufficient. Sir J. Herschel, the inventor of this 

 plan, employed it with a Newtonian reflector at the Cape, and 

 Dawes applied it to the achromatic immediately afterwards. 

 The reflection from the second surface must be got rid of, to 

 avoid the doubling of the image. This may be effected, by 

 roughening the back, or by using a prism — a modification 

 adopted by Cooke with his large achromatics. The end of the 

 tube, behind the " transparent diagonal," should be open to 

 admit of the free escape of the transmitted heat. This method is 

 so effectual, that substituting a piece of transparent glass for 

 the small speculum in his great Newtonian silvered reflector, 

 Mr. Bird was able to use his 12-inch aperture for nearly an 

 hour without inconvenience, oven during the intense heat of 

 last May. 



The most, remarkable apparatus, however, and that which 

 has Led hitherto to tin; greatest discoveries, is " Dawes's Solar 

 Eye-piece," so named from the eminent observer who invented 

 it. In this arrangement, a metal slide is perforated with a 

 'lies of holes, varying in size from 0*5 (or 0"8, which is safer 

 I'nv the screens) down to 0*0076 of an inch, any one of which 

 may be brought into the centre of the field at pleasure. The 



greater part of the heat is so completely intercepted by the 

 metal, which is turn is insulated by a plate of ivory from com- 

 iiiuniciition with tlie eye-piece, that tho inventor has used this 

 simple arrangement on Lassell's great 24-inch speculum for two 

 hours of bright sunshine without unpleasantly heating the eye- 



